Reduced graphene oxide-catalyzed oxidative coupling reaction of 4-methoxyphenol in aerobic aqueous solution

Abstract

Three reduced graphene oxide (rGO) with different defect contents were prepared at 250, 600 and 1000 °C, respectively, using thermal reduction method, and their effects on the oxidative coupling processes of 4-methoxyphenol (MOP) in aqueous solution were investigated. In the presence of rGOs, the oxidation of MOP was dramatically accelerated. At the same surface area dose, the catalytic efficiencies of three rGOs followed an ascending order of rGO250 ≤ rGO600 < rGO1000. Using liquid chromatography-mass spectrometry and molecular modeling computation, we identified the coupling products (dimer, trimer, and tetramer), and revealed that ortho CO and ortho-ortho CC coupling between two MOP radicals were the main reaction pathways. Dissolved oxygen and structural defects of rGOs played important roles in the oxidation of MOP. It is proposed that dissolved oxygen reacted with the defect sites of rGO to produce surface-bound superoxide species, which captured H atom from the phenolic hydroxyl group of MOP to generate MOP radical and H2O2. A portion of H2O2 can be catalyzed further by rGO to hydroxyl radical, which continued to react with MOP to produce MOP radical and H2O.